1. Field of the Invention
The present invention relates to a phase locked loop ("PLL"), and more particularly, to a frequency detector which generates a frequency error signal for controlling the oscillation frequency of a voltage-controlled oscillator ("VCO") in the PLL.
2. Description of the Related Arts
Typically, the reproduction of an Eight-to-Fourteen Modulation ("EFM") signal of a Compact Disc ("CD") player, for example, requires a bit clock synchronized to the EFM signal. This bit clock is generated in the PLL of the CD player. The EFM signal comprises various frequencies ranging from a minimum active duration Tmin of 3 T (where T denotes a system clock period) to a maximum active duration Tmax of 11 T.
The oscillation frequency of a VCO in a PLL circuit is typically controlled using a bit clock. To generate a bit clock in a conventional PLL, the number of clocks generated from the VCO in the PLL circuit are counted during the maximum duration Tmax of the EFM signal. That is, the oscillation frequency of the VCO is controlled according to the difference between the number of VCO clocks counted during the maximum active duration Tmax of the VCO and the number of VCO clocks during a normal operation.
The counting method described above requires a VCO which oscillates at a much greater frequency than the oscillation frequency of the EFM signal. Notably, it is difficult to implement a VCO which oscillates at a high frequency in an integrated circuit. Also, since the oscillation characteristics of the VCO are not linearly related to the control voltage, more accurate correction is required at higher frequencies.
Furthermore, as the frequency of the EFM signal is increased according to the reproduction speed of the CD, the higher speed CD player requires a VCO oscillating at an even higher frequency.
Also, counting the VCO clocks is carried out intermittently because VCO information is generated once per 588 T frame. Accordingly, where the frequency of the EFM signal changes rapidly, in a variable-speed reproduction operation, for example, the oscillation frequency of the VCO cannot rapidly follow the change of the EFM signal frequency.